1. Field of the Invention
This invention relates to continuous thickeners, clarifiers and similar gravitational settling devices for separating feed slurries or pulps into clarified liquid and sludge and is particularly concerned with a method for controlling the operation of such devices to improve their efficiency.
2. Description of the Prior Art
Continuous thickeners, clarifiers and similar gravitational settling devices are widely used in the chemical and metallurgical industries for the removal of liquids from slurries, metallurgical pulps, sewage, and other liquid-solid suspensions. Such devices generally include a circular tank having a cylindrical center well which extends downwardly into the vessel and is open at the bottom. The incoming slurry or pulp passes through a feed pipe or launder into the upper part of this center well and is introduced into the surrounding liquid through the bottom of the well in a manner designed to create a minimum of turbulence. This makes it possible to contain the bulk of the solids near the center of the unit. On leaving the well, the liquid entering with the pulp or slurry tends to move outwardly in a radial direction and flow upwardly toward a peripheral overflow launder. The solids suspended in the slurry or pulp settle downwardly through the slow-moving liquid and accumulate on the bottom of the unit. These solids are compressed as they accumulate and are slowly moved toward a bottom sludge discharge opening by means of slowly revolving rakes suspended a short distance above the bottom. The rakes aid in compressing the sludge and reduce its liquid content to a greater extent than is normally achieved in batch sedimentation operations.
During the normal operation of a thickener, clarifier or similar continuous gravity sedimentation device of the type referred to above, a series of relatively well-defined, vertically-spaced zones exist within the unit. The uppermost of these zones comprises a layer of clear liquid or clarified solution from which most of the solids have settled out. Below this is an intermediate layer containing suspended solid particles which is generally referred to as the "floc" layer. The interface between the clear solution and the floc layer is normally referred to as the "slime level". At the bottom of the unit is a layer of settled sludge. Such a system is a dynamic one characterized by the movement of liquid and solid particles between these zones. The levels of the three zones may vary considerably, depending upon the feed stream, operating conditions and other variables. During startup of the unit and during periods in which significant changes in feed rate, feed composition or other variables take place, three distinct zones may not exist. To achieve maximum capacity with such a unit, it has generally been thought that the slime level should be maintained as close to the top of the unit as possible and that only a thin layer of clarified solution should be maintained above the floc layer.
It is conventional to add flocculants or coagulants to thickeners, clarifiers and similar devices to increase their capacities. These materials cause the suspended particles in the slurry or pulp to flocculate or agglomerate and thus settle more rapidly. The amount of flocculant or the like which is required at any particular moment depends in part upon the slurry or pulp feed rate, the solids content of the feed, the solids size range and distribution, the densities of the solid particles, and the temperature and other operating conditions. Under constant conditions, the amount of flocculant needed to achieve maximum capacity in a particular thickener, clarifier or the like can generally be determined by trial and error. In actual practice, however, the conditions may change rapidly due to variations in the amount and composition of the solids suspended in the feed stream and other variables over which the operator of the unit may have relatively little or no control. Frequent adjustment of the amount of flocculant or coagulant added to the system is necessary to compensate for these variations and maintain the desired capacity and degree of separation while at the same time keeping operating costs within acceptable bounds by eliminating overflocculation.
It is common practice to use the slime level within a thickener or clarifier as a measure of the unit's performance and to monitor this level as a means for determining the need for changes in the flocculant rate. In general, the higher the slime level, the more flocculant that is needed. This measurement of the slime level has generally been done manually by means of measuring sticks lowered into the vessel near the outer edge of the unit. It has also been proposed that floats, differential pressure cells, or radiation detectors be positioned in the outer part of the tank to locate the step discontinuity in density representing the slime level and that changes in this level be used to control the addition of flocculant to the system. The slime level is not a direct measure of the settling characteristics of the solids in the pulp or slurry, however, and instead is the effect of a combination of variables, including flocculant rate, solids feed rate, solids and liquid characteristics, and the like. There is normally a long time lag between changes in the flocculant rate and corresponding changes in the slime level and hence the operator must estimate the amount of change in the flocculant rate which will be needed to produce a desired change in slime level. If he overestimates or underestimates the change in rate required, the unit may become unstable and eventually have to be shut down to avoid overloading or the carryover of slimes. The slime level therefore provides at best a visible means for assessing the state of the thickener or clarifier operation and, if it increases progressively, may serve as an advance warning that the capacity of the thickener or clarifier has been exceeded. It is virtually useless as a means for controlling the rate of flocculant addition.
Recognition that earlier methods for controlling thickener operation by regulating the rate of flocculant addition are ineffective has led to a suggestion that the incoming feed slurry or pulp in the feed launder be sampled at regular intervals downstream of the point of flocculant addition, that each sample thus collected be passed into a special gravity separation vessel where representative settling can take place, and that the separated liquid and solids phases be separately withdrawn from this special separation vessel in a volume ratio determined by the conditions desired in the thickener or clarifier. By sensing the interface level between the liquid and solids phases in the separation vessel and adjusting the rate of flocculant or coagulant addition to the feed stream in accordance with variations in the level of the interface during operation of the system, it has been said that the rate of flocculant addition can be controlled automatically and that the flocculant consumption can be reduced substantially. Experience has demonstrated, however, that the system thus proposed is not effective. Its use has therefore been abandoned. Efforts to develop other, more reliable methods for the automatic control of flocculant or coagulant addition in order to stabilize the operation of thickeners and clarifiers and reduce chemical consumption have in the past been largely unsuccessful.